Amplitude limiter



June 13, 1950 J VAN SLOQTEN 2,511,656

AMPLITUDE LIMITER Filed April 19, i946 lllllll Illllll 04 l l 1 ml v2-lllllll l lllllll l lllllll j .100 .710 10000 JACOB VAMSLOOZFN INVENTOR.

ATTORNEY Patented June 13, 1950 UNITED STATE PATENT oFFlcE'f'I AMPLITUDEny'n'rurt Jacob van Slooten, Eindhoven, Netherlands, assignor, by mesneassignments, toHartford National Bank and Trust Conn., as trusteeCompany, Hartford,

Application April 19, 1946, Serial No. 663,428

In the Netherlands November 6, 1941 Section 1, Public Law 690, August8,1946 Patent expires November 6, 1961 6 Claims. (01. 179 171) or beyondthe cut-off point. If the operating point is located exactly at thecut-ofi-point, then the positive wave halves of the incomingoscillations are still fully active. However, beyond this cut-off point,the flow of anode current is, to

an increasing extent, caused only by the positive voltage peaks becominggradually narrower. Thus, the phenomenon occurs that, with a gradualincrease of input voltage, the output voltage of the tube at firstslowly increases, then remains sequently starts to decrease. This doesnot satisfy the principle requirement of a good limiter, which is thatthe output voltage must remain substantially constant above a definitevalue of the input voltage. The drawback is caused by the fact that theamplitude of the active incoming positive voltagev peaks is maintainedapproximately constant, but the angle 2 as a part of the period 21rduring which these peaks are present, becomes smaller with an increasedin input voltage, due to these peaks becoming gradually narrower, asabove described. The mean anode current thus also decreases, as well asthe amount of energy per cycle in the output circuit.

The present invention has for its object a simple means whereby thisdrawback is obviated. It consists in so choosing the operatingconditions of the tube that, within the control region of the inputvoltages, the decrease of output voltage which with an increase in inputvoltage would be due to the mean grid voltage becoming more stronglynegative, is approximately compensated.

These conditions may be obtained in different manners, for instance by acorrect choice of the grid leak resistance which determines theinstantaneous value of the grid bias, and by including a resistance inthe cathode lead having such a value that, with an increase in signalintensity,

the mean anode current may be maintained substantially. constant due tolower voltage drop in this resistance and higher voltage drop in thegrid leakage resistance.

If the discharge tube used is a pentode or other be. obtained, it isadvantageous to choose the mean anode voltage to be applied of suchvalue that the rise of this voltage in itself entails also an increaseof the mean anode current.

; Inorder to cause the anode voltage to increase with an increase innegative grid-bias, it is of course necessar that the external anodecircuit should have a certain resistance. In the case of "the conductorleading to the screen grid so that,

a screengri'd tube it is possible to connect in parallel thereto, orinstead thereof, a resistance in with an increase in signal intensityand consequently an increase in negative bias of the control grid, thescreen grid potentialwill increase and contribute to maintainingconstancy of the 'mean anode current, which in itself would decrease dueto the control-grid voltage becoming .more negative.

As a matter' of fact, the invention is based on :20 the recognition, ofthe fact that limiter circuits substantially constant for a short time,but sub-"" drawing grid current exhibit a good limiter action only ifcare is taken to insure that the mean anode ,current remainssubstantially constant within the control region, since in thes circuitsthis will also be'f'the case with the alternating current component'(at'least with signals not too weak).

According to the ,invention, in the case of a pentode use may alsoadvantageously be made of .an adjustable potential on the third orsuppressor grid, in order to adjust the value to which the outputvoltage is limited.

In order that the invention may be more clearly understood and readilycarried into effect, it will be described more fully b reference to theaccompanying drawing forming a part of the specification and in which:

Fig. 1 shows agraph of the output voltage as a function .of the inputvoltage at different values of the'grid resistanc for a grid-leaklimiter circuit, such as shown in Fig. 2,

Figs. 2 and 3 aretwo different embodiments of the invention, and

Fig. 4 shows a few dynamic characteristic curves of the circuit shown inFig. 3.

v The curves I to VI shown in Fig. 1 give the variation of the outputvoltage V0 of a grid- .continuesto rise, curve II at first remainssubstantially plane and then rises further, whereas tube in which veryhigh internal. resistances 111 3157 5 1 curves I1I--V1in itially xhibita drop whichconspectively are, however, so small that the circuit:

cannot be used in a conjunction with a tuning circuit of good quality,since such a circuit would be far too strongly damped. With thehighervalues which may be used in this respect for the leakage resistance, forexample 500,000 to 600,000 ohms (curve V) the output voltage declinestoo much as a result of the reasons above-described.

In the circuits of Figs. 2 and 3 which ar applicable to frequencmodulation receivers and which will now be described care has been takenin different manners to insure that a satisfactory tance triode ortriode-connected tube will be satisfactory. The desired effect isobtained by connecting a high resistance 3 bridged by a condenser 2 inthe supply conductor leading to the cathode of the tube. The order ofmagnitude of this resistance is 10,000 ohms, whereas that ofthe leakageresistance 4 may amount to 500,000 ohms or higher. By means of a sourceof supply 5, the bias for the grid must in this case be made positiverelatively to the earthed end of the resistance 3, since the totalvoltage drop in this resistance may easily amount to 50 volts andhigher.

In this case the control voltage for automatic volume control may bederived from the grid end of the resistance 4 in the usual manner via aresistance 6.

Due to the presence of the high cathode resistance 3, the mean anodecurrent and hence the output voltage may be maintained sufficientlyconstant and the tube operates very satisfactorily as a limiter. v

In the construction shown in Fig. 3 use is made of a pentode tube 1having a grid leakage resistance 8, a screen-grid resistance 9 and ananode series-resistance [0. third grid is made variable. In the circuitof Fig. 3 a tube type EF50 has given satisfactory results. Anotherexample of a suitable tube is a type EF42. The value of the resistanceIn is now so chosen that the mean anode voltage is so low thatvariations in this voltage still'bring about considerable variations inthe mean anode current and vice versa. Consequently, with an in creasein signal intensity, in which case the negative bias on the control gridrises, the anode voltage will increase and counteract the eifect of thecontrol grid upon the mean anode current. The latter may consequently bemaintained substantially constant and the same applies to the outputvoltage. Moreover, a resistance 9 is provided in the screen-gridcondoctor so that not only the mean anode potential but also thescreen-grid voltage will increase with the intensity of the signal andcontribute to maintaining constancy of the mean anode current.

Fig. 4 illustrates graphically the limiter action of this circuit on adouble logarithmic scale. In this case the resistance'B hadta value of600,000

"ohms and the resistances 9 and I value of The voltage on the i 100,000and 64,000 ohms respectively. The voltage of the anode battery amountedto volts. Curve A represents the variation of the output voltage V0 as afunction of the input voltage Vi with a bias on the third grid of +4volts, Whereas curve B represents the same variation with 6 volts.Consequently, variation of thi bias apparently only results in adisplacement of the horizontal portion of the curve parallel to itselfso that, with the aid of this Variation, the adiustment of the value towhich limitation of the output voltage is desired is possible withindefinite limits.

I claim:

1. A circuit arrangement for limiting the amplitude of a radio frequencysignal voltage, comprising an electron discharge tube having an inputcircuit including a cathode electrode and a control electrode and ananode electrode, and an outputcircuit including said cathode, means toapply operating potentials to said tube, means to apply said signalvoltage to the input circuit, means to bias the control electrode ofsaid tube below the cut-off value, said means comprising a grid-leakresistor interposed between the control electrode and cathode electrodeof said tube and having a value at which successive increases of thesignal voltage above a given value normally produce a decrease in thecathode-anode current, a resistance element in said output circuithaving a value at which the voltage drop across the said resistanceelement establishes an operating point of said tube at which the saiddecrease in the cathode-anode current is substantially neutralized, andmeans to by-pass said latter resistance element for said signal voltage.

2. A circuit arrangement for limiting the amplitude of a radio frequencysignal voltage, comprising an electron discharge tube having an inputcircuit including a cathode electrode and a control electrode, and anoutput circuit including said cathode and an anode electrode, means toapply operating potentials to said tube, means to apply said signalvoltage to said input circuit, means to bias the control electrode ofsaid tube below the cut-off value, said means comprising a grid-leakresistor. interposed between the control electrode and cathode electrodeof said tube and having a value, at which successive increases of thesignal voltage above a given value normally produce a decrease in thecathode-anode current, a cathoderesistor coupled to the cathode of saidtube, and having a value at which a screen electrode andan anodeelectrode, means to apply operating potentials to said tube, meanstofiapply', said signal voltage to said input circuit, means to bias.the control grid of said tube below the cut-ofi' v-allle,v said meanscomprising a gridleak resistor interposed between the control grid andcathode of said tube and having a value at which successive increases ofthe signal voltage above a given value normally produce a decrease inthe cathode-anode current, a'resistance elementin said outputc'ircu'itinterposed between said operating potential application means and one ofsaid electrodes and having a value at which the voltage drop across thesaid resistance element establishes an operating point of said tube atwhich the said decrease in the cathodeanode current is substantiallyneutralized, and means to by-pass said latter resistance element forsaid signal voltage.

4. A circuit arrangement for limiting the amlitude of a radio frequencysignal voltage, comprising an electron discharge tube having an inputcircuit including a cathode and a control grid and an output circuitincluding said cathode, a screen grid and an anode, means to applyoperating potentials to said tube, means to apply said signal voltage tosaid input circuit, means to bias the control grid of said tube belowthe cut-off value, said means comprising a grid-leak resistor interposedbetween the control grid and cathode of said tube and having a value atwhich successive increases of the signal voltage above a given valuenormally produce a decrease in the cathode-anode current, a, resistanceelement in said output circuit interposed between said anode and theanode potential application means and having a value at which thevoltage drop across the said resistance element establishes an operatingpoint of said tube at which the said decrease in the cathode-anodecurrent is substantially neutralized, and means to bypass said latterresistance element for said signal voltage.

5. A circuit arrangement for limiting the amplitude of a radio frequencysignal voltage, comprising an electron discharge tube having an inputcircuit including a cathode and a control grid and an output circuitincluding said cathode, a screen grid and an anode, means to applyoperating potentials to said tube, means to apply said signal voltage tosaid input circuit, means to bias the control grid of said tube belowthe cut-off value, said means comprising a. grid-leak resistorinterposed between the control grid and cathode of said tube and havinga value at which successive increases of the signal voltage above agiven value normally produce a decrease in the cathode-anode current, aresistance element in said output circuit interposed between saidoperating potential application means and said screen-grid and having avalue at which the 50 voltage drop across the said resistance elementestablishes an operating point of said tube at which the said decreasein the cathode-anode current is substantially neutralized, and means toby-pass said latter resistance element for said signal voltage.

6. A circuit arrangement for limiting the amplitude of a radio frequencysignal voltage, comprising an electron discharge tube having an inputcircuit including a cathode and a control grid and an output circuitincluding said cathode, a screen grid and an anode, means to applyoperating potentials to said tube, means to apply said signal voltage tosaid input circuit, means to bias the control electrode of said tubebelow the cut-off value, said means comprising a gridleak resistorinterposed between the control grid and cathode of said tube and havinga value at which successive increases of the signal voltage above agiven value normally produce a decrease in the cathode-anode current, anoutput impedance element coupled to the anode of said tube, a firstresistance element in said output circuit interposed between said outputimpedance element and said potential application means and having a,value at which the voltage drop across the said resistance elementestablishes an operating point of said tube at which the said decreasein the cathode-anode current is substantially neutralized, a secondresistance element interposed between said potential application meansand said screen-grid and having a value at which successive increases ofsaid signal voltage above said given value successively increases thescreen-grid potential to aid in neutralization of said decrease in thecathode-anode current, and means to by-pas-s said first and secandresistance elements for said signal voltage.

JACOB vm SLOOTEN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UN STATES PATENTS Number Name Date 1,798,008 Bussard Oct. 23, 19342,022,972 Nebel Dec. 3, 1935 2,357,405 Katzin Sept. 5, 1944 FOREIGNPATENTS Number Country Date 114,689 Australia Feb. 12, 1942 vCertificate of Correction Patent No. 2,511,656 June 13, 1950 JACOB VANSLOOTEN It is hereby certified that errors appear in the printedspecification of the above numbered patent requiring correction asfollows:

Column 4, lines 19 and 20, strike out the Words control electrode and ananode electrode, and an output circuit including said cathode, means andinsert instead control electrode, and an output circuit including saidcathode and an anode electrode, means; a V and that the saidLettersiPatent should be read with these corrections therein that thesame may conform to the record of the case in the Patent Office.

Signed and sealed this 26th day of September, A. D. 1950.

[SEAL] THOMAS F. MURPHY,

Assistant Oommissz'oner of Patents.

